Oxidative stress is defined as an imbalance between the oxidative damages of reactive oxygen species produced in vivo and the antioxidant potential of biological antioxidative system. Originally, reactive oxygen species are useful products which are formed during energy generation, xenobiotic attack, disposal of unnecessary cells, cellular signaling, etc. Once excessive reactive oxygen species unable to be scavenged by the biological antioxidative system are generated, however, these species oxidize the lipids, proteins or enzymes that serve the structures or functions in living organisms or genetic DNA carrying genetic information, cause damages, disrupt the structures or functions in living organisms, cause diseases such as cancer or lifestyle-related disease, or accelerate the aging process (Non-Patent Document 1).
Peroxides such as active oxygen produced in vivo when H2O is formed from the oxygen taken up during respiration in the mitochondrial electron transport system are thought to increase oxidative stress. It is known that mitochondria not only make up ATP to sustain cell life but also play a crucial role in aboptosis (cell death). Oxidized LDL, oxidized RLP, etc. are also known to increase oxidative stress in vascular endothelial cells.
As such, mitochondria are the source for generating reactive oxygen and on the other hand, mitochondria are one of organelles which are most vulnerable to oxidative stress. Therefore, analysis of oxidative stress in mitochondria is greatly helpful to elucidate the mechanisms of many diseases associated with oxidative stress. For this reason, it is extremely useful to produce functional molecules for visualizing the level of oxidative stress in mitochondria.
Peroxide scavengers have been developed so far but no reagent for exclusively scavenging peroxides alone has been developed (Non-Patent Document 2). No peroxide scavenger localized in mitochondria has been developed, either (Non-Patent Document 3).    [Non-Patent Document 1] Web site of Japanese Society of Antioxidants (http://www.jsa-site.com/sanka_storesu.htm)    [Non-Patent Document 2] Suzuki, B., et al., Bioorg. Med. Chem. Lett. 2007, 2055-2058    [Non-Patent Document 3] Okimoto, Y., et al., FEBS Lett., 2000, 474, 137-140